Genetic Analysis of Proteoglycan-Mediated Lipoprotein Clearance. Hypertriglyceridemia results from the accumulation of triglyceride-rich lipoproteins (TRLs) in the circulation (chylomicrons, very low-density lipoproteins, and their remnants). Because patients with hypertriglyceridemia have increased risk for atherosclerosis and coronary artery disease, considerable interest exists in understanding its etiology and therapy. TRL accumulation can arise from altered lipid biosynthesis, apolipoproteinemias, and alterations in lipolysis in the peripheral circulation. It can also arise from defective clearance of lipoprotein remnants in the liver, which occurs through a multi-step process involving sequestration of remnant lipoproteins in the space of Disse and receptor-mediated endocytosis by hepatocytes. Hepatocytes express several receptors, including members of the LDL receptor family and Syndecan-1 (Sdc1), a trans-membrane heparan sulfate proteoglycan. Studies of Sdc1-deficient mice showed that Sdc1 is the primary proteoglycan receptor that mediates TRL clearance. Additionally, hepatocyte-specific inactivation of the heparan sulfate biosynthetic enzymes, GlcNAc N-deacetylase/N-sulfotransferase-1 (Ndst1) and uronyl 2-O-sulftotransferase (Hs2st) using the Cre-loxP system demonstrated the importance of the heparan sulfate chains of Sdc1. Mutant mice bearing defects in heparan sulfate biosynthesis and Sdc1 provide a model for defining genes relevant to hypertriglyceridemia in humans. To better understand how Sdc1 and heparan sulfate structure relate to lipoprotein clearance in the liver, we have the following specific aims: Aim 1. Determine the structural features of Syndecan-1 that facilitate its action as a lipoprotein receptor in vivo. Aim 2. Explore the structural features of heparan sulfate required for lipoprotein clearance in vivo. Aim 3. Characterize the impact of mutations on Syndecan-1 structure and lipoprotein binding in vitro. Aim 4. Investigate the relative contribution of Syndecan-1 to hepatic clearance of triglyceride-rich lipoproteins in human and mouse hepatocytes. Our genetic analysis of HSPGs in the mouse support the idea that changes in liver heparan sulfate could be an underlying cause of human dyslipidemias. Thus, determining the relevant genes involved in HSPG synthesis in the liver and their functional role in remnant clearance provides a series of candidate genes for eventual allelic analysis in patients with hypertriglyceridemia.